The present invention relates generally to the fields of immunology and molecular biology, and particularly to a polypeptide growth factor which regulates the proliferation and differentiation of lymphocyte and other hematopoietic progenitors.
B and T lymphocytes are the primary effector cells of the immune responses. Both cell classes are considered to derive ultimately from hematopoietic stem cells in the mammalian bone marrow, via progenitor or precursor cells representing distinguishable stages in the differentiation of each class.
B lymphocytes, or B cells, are the precursors of circulating antibody-secreting plasma cells. Mature B cells, which are characterized by expression of surface-bound immunoglobulin capable of binding specific antigens, derive from hematopoietic stem cells via an intermediary cell class known as pre-B cells. Mature T cells develop principally in the thymus, presumably from an as-yet-unidentified precursor cell which migrates from the bone marrow to the thymus at an early stage of T lymphocyte development.
While considerable progress has been made in identifying soluble hormone-like factors which regulate differentiation of other cells of hematopoietic origin, for example, granulocytes and macrophages, very little is known about regulatory factors involved in B and T cell lymphogenesis. The pluripotent lymphoid stem cell has not been identified, nor have all of the factors or conditions required for commitment and expansion of the B and T cell lineages been defined. The later stages of B and T cell growth and differentiation, following the appearance of surface immunoglobulin and the emergence of the B cell from the bone marrow, or the T cell from the thymus, have been the most well studied. This work has revealed a number of factors which are active on mature peripheral B and T cells, including IL-1, IL-2, IL-4, IL-5, interferon gamma, BSF-2, neuroleukin, and transforming growth factor beta.
Fragmentary evidence has been made available concerning the B cells considered to be the immediate precursors of mature functional peripheral B cells. These pre-B cells have been defined as cells containing cytoplasmic .mu. chain but no cytoplasmic light chain and no surface immunoglobulin.
Paige, Nature 302:711 (1983) and Paige et al., Eur. J. Immunol. 14:979 (1984) described techniques for culturing B cell progenitors derived from 14-day murine fetal liver cells. These pre-B cells were observed to differentiate to antibody secreting B cells in vitro when cultures were supported by a layer of fetal liver cells, bone marrow adherent cells, or conditioned media containing colony stimulating factors.
Giri et al., J. Immunol. 132:223 (1984) cultured a murine bone marrow-derived pre-B cell line, 70z/3, in the presence of partially purified IL-1 preparations derived from lipopolysaccharide (LPS) induced P388D1 cell supernatants. Following contact with the IL-1 preparation, the pre-B cells were observed to express surface immunoglobulin. Jyonouchi et al., J. Immunol. 135:1891 (1985) reported that a humoral factor or factors from the serum of NZB mice could enhance the maturation of B lineage precursor cells. Landreth et al., J. Immunol. 134:2305 (1985) demonstrated that a factor (or factors) present in the urine of a cyclic neutropenic patient stimulated generation of pre-B cells in human and mouse bone marrow cultures.
However, these results have been difficult to interpret due to the non-homogeneous nature of the cell populations involved. The pleiotropic effects of known lymphokines, as well as their potent intrinsic biological activities, complicate analysis of assays involving the response of heterogeneous cell cultures to conditioned cell culture media derived from serum-containing cell cultures.
One major impediment to the study of lymphocyte development has been the difficulty associated with obtaining enriched populations of viable lymphoid precursors. The long term bone marrow culture system developed by Whitlock and Witte, J. Immunol. Meth. 67:353 (1984) has provided a source of pre-B cells for study. In a Whitlock-Witte culture, an adherent support layer comprising stromal-derived fibroblasts, macrophages, and endothelial cells is employed as a feeder layer to support the growth of an upper phase of nonadherent pre-B and earlier lymphoid precursors.
Whitlock et al., Cell 48:1009 (1987), and Hunt et al., Cell 48:997 (1987) reported cloning of cell lines derived from murine bone marrow stroma which were capable of supporting growth of pre-B cells and other precursors of mature B cells in Whitlock-Witte cultures. Supernatants of one such cell line, designated ALC, were tested for activity in IL-1, IL-2, IL-3, and IL-4 assays. The absence of activity in these assays suggested that these factors were not the source of the proliferation-inducing activity found in ALC supernatants. Both Whitlock et al. and Hunt et al. considered the pre-B cell inducing activity which they observed to be attributable to a novel factor present in the stromal cell supernatants.
In developing the present invention, a variant of the Whitlock-Witte culture system was employed as a source of immature B cells, which were the basis for a rapid quantitative assay for detecting growth factors capable of stimulating proliferation of pre-B cells. A putative factor observed in murine stromal cell cultures was tentatively designated "Lymphopoietin-1" ("LP-1"), and subsequently designated "Interleukin-7" ("IL-7"). To clone a murine IL-7 cDNA, which was used to secure the human cDNA, a new cloning technique was employed. To obtain a cell line which expressed a soluble murine IL-7 in detectable quantities, a novel cell line was established by transformation of stromal cells derived from a Whitlock-Witte culture. This cell line, which secretes a pre-B cell growth activity in serum-free media, provided specific IL-7 messenger RNA for expression cloning, and protein for purification and sequencing. Isolation of a murine cDNA clone enabled identification by cross-hybridization of human genomic and cDNA clones encoding human IL-7.
Preliminary experiments involving administration of purified recombinant IL-7 to mice have indicated that in addition to stimulating the development and proliferation of the hematopoietic precursors of T and B cells, IL-7 is also capable of inducing the proliferation of megakaryocyte and granulocyte/macrophage precursors in bone marrow. In view of potential immunological therapeutic uses in stimulating proliferation of precursors of T cells and antibody-secreting B cells, as well as other hematopoietic cell types, there is considerable interest in developing technology for producing biologically active IL-7 molecules. Use of recombinant expression systems can provide sufficient quantities of pure protein to permit detailed study of the biological activity of such molecules and to supply anticipated clinical demands for therapeutic IL-7 compositions.